使用java.time 包中的類替代舊的Date 和Calendar 類；2. 通過(guò)LocalDate、LocalDateTime 和LocalTime 獲取當(dāng)前日期時(shí)間；3. 使用of() 方法創(chuàng)建特定日期時(shí)間；4. 利用plus/minus 方法不可變地增減時(shí)間；5. 使用ZonedDateTime 和ZoneId 處理時(shí)區(qū)；6. 通過(guò)DateTimeFormatter 格式化和解析日期字符串；7. 必要時(shí)通過(guò)Instant 與舊日期類型兼容；現(xiàn)代Java 中日期處理應(yīng)優(yōu)先使用java.time API，它提供了清晰、不可變且線程安全的接口，適用於絕大多數(shù)實(shí)際場(chǎng)景，並能有效避免傳統(tǒng)日期類的缺陷。

Working with calendars in Java can be confusing if you're using outdated methods, but modern Java (especially from Java 8 onward) provides a much cleaner and intuitive way to handle dates and times via the java.time package. Here's a practical guide on how to work with calendars effectively.

? Use java.time Instead of Old Date/Calendar Classes

Avoid the old java.util.Date and java.util.Calendar classes — they're mutable, not thread-safe, and hard to work with. Instead, use the java.time API introduced in Java 8 .

Key classes:

• LocalDateTime – Date and time without timezone
• ZonedDateTime – Date and time with timezone
• LocalDate – Just a date (eg, 2025-04-05)
• LocalTime – Just a time (eg, 14:30)
• YearMonth , MonthDay , etc. – Specialized date types

? 1. Get Current Date and Time

 import java.time.LocalDateTime;
import java.time.LocalDate;
import java.time.LocalTime;

LocalDateTime now = LocalDateTime.now();
System.out.println("Now: " now);

LocalDate today = LocalDate.now();
System.out.println("Today: " today);

LocalTime time = LocalTime.now();
System.out.println("Current time: " time);

This gives you clean, immutable objects representing the current moment.

?? 2. Create Specific Dates

You can create fixed dates easily:

 LocalDate birthday = LocalDate.of(1990, 5, 15); // Year, Month, Day
// Or use Month enum
LocalDate xmas = LocalDate.of(2025, java.time.Month.DECEMBER, 25);

Similarly for time:

 LocalTime meetingTime = LocalTime.of(14, 30); // 2:30 PM

Combine date and time:

 LocalDateTime event = LocalDateTime.of(2025, 3, 20, 9, 0); // Mar 20, 9:00 AM

? 3. Add or Subtract Time (Immutably)

All java.time objects are immutable , so you get a new instance when modifying.

 LocalDate today = LocalDate.now();
LocalDate nextWeek = today.plusWeeks(1);
LocalDate nextMonth = today.plusMonths(1);
LocalDate tenDaysAgo = today.minusDays(10);

System.out.println("Next week: " nextWeek);

Works similarly for LocalDateTime , LocalTime , etc.

? 4. Work With Time Zones (ZonedDateTime)

If you need time zones (eg, scheduling across regions):

 import java.time.ZoneId;
import java.time.ZonedDateTime;

ZoneId nyZone = ZoneId.of("America/New_York");
ZonedDateTime nyTime = ZonedDateTime.now(nyZone);

ZoneId londonZone = ZoneId.of("Europe/London");
ZonedDateTime londonTime = ZonedDateTime.now(londonZone);

System.out.println("NY: " nyTime);
System.out.println("London: " londonTime);

You can also convert between zones:

 ZonedDateTime nyInLondonTime = nyTime.withZoneSameInstant(londonZone);

? 5. Format and Parse Dates

Convert to/from strings using DateTimeFormatter .

 import java.time.format.DateTimeFormatter;

LocalDate date = LocalDate.of(2025, 4, 5);
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");
String formatted = date.format(formatter); // "05-04-2025"
System.out.println(formatted);

// Parse string back to date
String input = "15-08-2023";
LocalDate parsed = LocalDate.parse(input, formatter);

Common patterns:

• "yyyy-MM-dd" → 2025-04-05
• "dd/MM/yyyy HH:mm" → 05/04/2025 14:30
• "MMM dd, yyyy" → Apr 05, 2025

? 6. Legacy Compatibility (if needed)

If you must interact with old APIs that use java.util.Date :

 import java.util.Date;
import java.time.Instant;

// Convert LocalDateTime to Date
LocalDateTime ldt = LocalDateTime.now();
Date oldDate = Date.from(ldt.atZone(ZoneId.systemDefault()).toInstant());

// Convert Date to LocalDateTime
Date legacyDate = new Date();
LocalDateTime ldt2 = legacyDate.toInstant()
                               .atZone(ZoneId.systemDefault())
                               .toLocalDateTime();

But try to stick with java.time throughout your app.

? Pro Tips

• Always specify time zones explicitly when dealing with real-world events.
• Use Instant for timestamps (eg, logging, database storage).
• For recurring events or calendar logic (eg, "first Monday of the month"), consider libraries like ThreeTen-Extra or Joda-Time (though Joda is now in maintenance mode).

Basically, working with calendars in Java is clean and safe if you use java.time . Avoid the old Calendar class unless you're maintaining legacy code. Focus on LocalDate , LocalDateTime , ZonedDateTime , and formatting tools — they cover most real-world use cases.

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